Mechanized, linear motion can be achieved through a number of linear drive means including hydraulic cylinders, pneumatic cylinders, chain systems, rack and pinion systems, cable systems and lead screws assemblies. Lead screws assemblies are the best choice for effecting linear motion when the following are required: positional accuracy, design adaptability, simplicity of design, compactness of design, control compatibility, reliability, repeatability, ease of maintenance and high load capacity. An excellent discussion of the historical development of lead screws and the present designs thereof is provided in Lead Screws: A Technical Brief, (1989), available from Ball Screws & Actuators Co., Inc. of San Jose, Calif., the disclosure of which is incorporated herein by reference.
Modern lead screw assemblies generally comprise a lead screw and a nut assembly supported in bearings. All that is required to produce linear motion with a lead screw assembly is rotation of the lead screw or rotation of a follower nut. This simplicity enables the use of virtually any type of drive mechanism. Moreover, because the drive mechanism can be connected directly to the lead screw, precise control of motion is easily achieved. Lead screws are ideally suited for machine tools utilizing either manual or computer numerical control (CNC).
Linear guidance slideways (also known as guideways or raceways), ball bearings, and lead screw assemblies or actuators have a common developmental history in that the development of early milling machines and other industrial machines required a combination of these devices to perform a variety of machining operations, such as milling, drilling, and grinding. In these industrial machines, linear guidance slideways formed a guidepath to allow stock to be machined against tools and/or cutting devices. As many of these devices required high precision and heavy stock removal, a lead screw actuator comprising a lead screw and nut was placed between and/or adjacent the linear guidance slideways to provide both positional control and a mechanical advantage. Generally, the nut was attached to the guidance slideway and the ends of the lead screw were secured in bearing blocks to allow rotation of the lead screw. Motors or large wheels were attached to the end of the lead screw to provide rotational leverage or force.
Early slideways utilized sliding friction between two pads, lubricated with grease and placed on either side of the slideways. As industry became more mechanized, accuracy, speed, and force requirements led to the development of higher performance sealed bearings and ball bearing slideways. These devices limited the point of contact between slideways, thereby reducing friction and allowing machine tool builders to tighten tolerances between slideways. The basic design incorporated small ball bearings housed in a precision-machined raceway. Long raceways were constructed to allow the ball bearings to slide along shaftways. Because there was less friction between parts, machine tools could be constructed within narrower tolerances, while providing faster motion and greater accuracy.
THK Co., Ltd. of Tokyo, Japan developed a process for grinding several different raceways along the faces of a long rectangular steel bar. The resultant bearings and slideways are extremely accurate and durable, but are also extremely expensive because of the capital equipment and technology necessary to manufacture them.
A less expensive bearing designed by the Bishop-Wisecarver Corp. of Pittsburg, Calif. incorporates a round, sealed ball bearing with an extra thick outer raceway ground into a V-shape. This track roller rolls along a roll-formed, steel V-shaped track. Limitations of the mounting and supporting structure of the track roller and the relatively low precision of the roll-formed track prevent this design from obtaining a broad range of applications. It is mainly used in material handling applications rather than in machine tools.
INA Linear Technik, Inc. (INA) of Germany developed a modified version of the Bishop-Wisecarver V-roller bearing. By grinding an arched raceway, INA produced a double-row, angular contact, ball bearing. This design allows a track roller to ride on precision ground shafting as opposed to a relatively low-precision, rolled, V-track. In addition, the double-row bearing geometry is more stable and more securely mounted. These improvements have allowed use of the INA bearing in numerous machine tool applications. To reduce fabrication costs of the INA track roller linear guidance systems, the guideway is a composite structure comprising an aluminum body having a channel in which a linear, hardened and ground steel rod is disposed.
Similar to early linear guidance slideways, early lead screw actuators employed sliding friction between the lead screw and the following nut. Lubricant was placed directly on the lead screw and lubricated the follower nut as it slid along the lead screw. Lead screws of this type, known as Acme lead screws, are still in use today. Such Acme lead screws are available, for example, from Nook Industries, Inc. of Cleveland, Ohio. A certain amount of play or "backlash" (axial free motion between the nut and lead screw) is required to prevent jamming in such Acme lead screws. This backlash limits the application of such devices to medium tolerance machines and manual backdriving to correct for the backlash. The term "backdriving" describes the occurrence of an applied load forcing either the lead screw or the follower nut to rotate back through the driving mechanism. Moreover, the frictional forces generated by Acme lead screw assemblies make these assemblies quite inefficient.
Like linear guidance raceways, modern lead screw assemblies take advantage of the ball bearing. Lead ball screw assemblies began to appear in the United States in the 1940's. These assemblies comprise a lead ball screw, which is a shaft member having an inner helical ball track, and a ball nut, which is a follower nut containing an outer helical track housing ball bearings. Lead ball screw assemblies are available, for example, from Ball Screws & Actuators Co., Inc. and THK Co., Ltd.
The three basic methods of manufacturing lead screws and follower nuts are thread cutting, thread rolling and thread grinding. Thread cutting is generally limited to Acme screws. The lead of rolled lead screws is typically found to vary over the length of the lead screw. Ground lead screws assemblies provide better lead accuracy and repeatability, but are much more expensive to fabricate than rolled lead screw assemblies. To achieve accuracies better than 0.003 in./ft., the use of a precision ground lead screw assembly is generally required.
Even the cost of relatively less precise rolled lead screw assemblies is not insubstantial, however. In an attempt to alleviate such cost constraints, derivative linear driving mechanisms have been developed. For example, a Uhing.RTM. rolling ring drive is available from Amacoil, Inc. of Aston, Pa. This ball bearing device incorporates six standard ball bearings canted to a fixed or variable lead angle. Three canted ball bearings are positioned on one bearing block, and three canted ball bearings are positioned on another bearing block. The canted ball bearings are spring-loaded against a simple, low-cost ground round shaft. As the shaft is turned, the bearing block moves along the shaft according to the lead angle or pitch angle of the bearings, similar to the movement of a nut along a shaft. The motion is very smooth and the components are very inexpensive. However, because there is no raceway and the accuracy is dependent on friction developed by spring-loading, slippage of the bearing block renders the device useless for machine tool design.
It is very desirable to develop a linear drive assembly that can achieve the accuracy of rolled and ground lead screw assemblies without the expense associated with the fabrication of such lead screw assemblies.